1. Field of the Invention
The present invention relates to a photomask, a focus measurement apparatus and a focus measurement method, which are used for lithography when forming a semiconductor device.
2. Description of the Related Art
In recent years, patterns for semiconductor elements are demanded strict size precision. In a projecting aligner (hereinafter called the “aligner”) used to form a pattern, which projects and exposes a mask pattern by way of reduction to form an image of the pattern on a semiconductor wafer, there arises a problem that the size of a resist pattern after development varies when an image-formed position, namely a focus position, shifts.
Hence, although it is preferable that the focus position of the aligner does not shift, the focus position varies in some degree in practice, requiring aligner management such that a shifted amount of the focus position of the aligner is measured at regular intervals to correct a difference in depth of focus. In addition, it is required that difference in the focus position when an actual product is formed by patterning is recognized and the difference is corrected for the next exposure.
A generally-used measurement method of the focus position is that measuring the sizes of the exposed patterns one by one while changing the focus. The focus position of an isolated residual pattern formed by a positive-type resist is obtained by making use of the fact that the size is the largest at the focus position.
Further, there is proposed a method specifying a defocus amount by measuring a positional difference amount varying in accordance with the focus variation by adding an assist pattern to a positional difference test mark, in Japanese Patent Application Laid-Open No. 2004-172600 (Patent document 1).
In the method measuring the size of the transferred pattern exposed by way of changing the focus, the focus position cannot be obtained by a single exposure, leaving a problem that the difference in the focus position when the pattern of a product is exposed cannot be obtained. Also, in the method of Patent document 1, the positional difference with respect to the focus variation is small, leaving a problem that the focus variation cannot be specified with high precision.